Electroluminescent devices, which may be further classified as either organic or inorganic, are well known in graphic display and imaging art. The benefits of organic electroluminescent devices, such as organic light emitting devices, include: high visibility due to self-emission; superior impact resistance; and ease of handling of the solid state devices. OLEDs, such as organic light emitting diodes, may have practical application for television and graphic displays, as well as in digital printing applications.
An OLED is typically a thin film structure formed on a substrate such as soda-lime glass. A light emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The semiconductor layers may be either hole-injecting or electron-injecting layers. The light emitting layer may be selected from any of a multitude of fluorescent organic solids. The light emitting layer may consist of multiple sublayers.
When a potential difference is applied across the device, negatively charged electrons move from the cathode to the electron-injecting layer and finally into the layer(s) of organic material. At the same time, positive charges, typically referred to as holes, move from the anode to the hole-injection layer and finally into the same light emitting organic layer. When the positive and negative charges meet in the organic material layer(s), they recombine and produce photons. The wave length--and consequently the color--of the photons depends on the electronic properties of the organic material in which the photons are generated.
In a typical matrix-addressed OLED display, numerous OLEDs are formed on a single substrate and arranged in groups in a grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time.
OLEDs have a number of beneficial characteristics. These characteristics include a low activation voltage (about 2 volts), fast response when formed with a thin light emitting layer, and high brightness in proportion to the injected electric current. Depending on the composition of the organic material making up the light emitting layer, many different colors of light may be produced, ranging from visible blue, to green, yellow and red.
OLEDs are susceptible to damage resulting from exposure to the atmosphere. The fluorescent organic material in the light emitting layer can be reactive. Exposure to moisture and oxygen may cause a reduction in the useful life of the light emitting device. The organic material is susceptible to reacting with constituents of the atmosphere such as water and oxygen. Additionally, the materials that typically comprise the cathode and anode may react with oxygen and may be negatively affected by oxidation.
One disadvantage of oxygen and moisture penetration into the interior of the OLED is the potential to form metal oxide impurities at the metal-organic material interface. In a matrix addressed OLED, these metal oxide impurities may cause separation of the cathode or anode from the organic material. Oxidation sensitive cathode materials such as Mg--Ag or Al--Li are especially susceptible. The result may be dark, non-emitting spots at the areas of separation due to a lack of current flow.
Edge shorting between the cathode and anode layers is a further problem currently affecting most conventional OLED displays. Edge shorting reduces the illumination potential of the display devices.
For the reasons set forth above, exposing a conventional OLED to the atmosphere, shortens its life. To obtain a practical, useable OLED, it is necessary to protect or seal the device, so that water, oxygen, etc., do not infiltrate the light emitting layer or oxidize the electrodes.
Methods commonly employed for protecting or sealing inorganic electroluminescent devices are typically not effective for sealing OLEDs. For example, when the "silicon oil method" sometimes used for sealing inorganic electroluminescent devices is used on an OLED, the silicon oil infiltrates the light emitting layer, the electrodes, and any hole-injecting or electron-injecting layers. The oil alters the properties of the organic light emitting layer, reducing or eliminating its light emission capabilities. Similarly, resin coatings that have been used to protect inorganic electroluminescent devices are not suited for OLEDs. The solvent used in the resin coating solution tends to infiltrate the light emitting layer, degrading the light emission properties of the device.
U.S. Pat. No. 5,505,985 issued to Nakamura, et al., ("Nakamura") teaches a process for depositing a film comprising an electrically insulating polymer as a protective layer on an outer surface of an organic electroluminescent device. Nakamura asserts that the polymers disclosed protect the device and have excellent electrical resistivity, breakdown strength and moisture resistance, while at the same time are transparent to emitted light. Nakamura also teaches that, when deposited by a physical vapor deposition (PVD) method, the protective layer formed by the polymer compound is pin-hole free. The sealing method taught by Nakamura, however, yields a moisture diffusivity too high to be useful for reliable OLEDs. Moisture levels as low as 1 ppm may damage an OLED.
Others have tried evaporated metal films to seal an OLED. However, to avoid pinholes, these films must be relatively thick, resulting in poor light transmission.
Accordingly, there is a need for a method for sealing an OLED without adding to its complexity and expense. There is also a need for a method and apparatus for sealing an OLED which can be easily integrated into existing fabrication methods. The present invention meets this need, and provides other benefits as well.